In recent years, with the rising awareness of global environmental protection, improvements in fuel efficiency have been strongly required for reducing automobile CO2 emissions. This has led to active attempts to reduce the thickness of automotive parts by strengthening steel sheets, which are materials for automobile body parts, to reduce automobile weight.
In order to strengthen steel sheets, solid solution-strengthening elements such as Si and Mn are added. However, these elements are more oxidizable than Fe. Therefore, in the case of manufacturing galvanized steel sheets and galvannealed steel sheets from high-strength steel sheets containing large amounts of these elements, there are problems below.
In usual, in order to manufacture a galvanized steel sheet, after a steel sheet is heated and annealed at a temperature of about 600° C. to 900° C. a non-oxidizing atmosphere or a reducing atmosphere, the steel sheet is galvanized. Oxidizable elements in steel are selectively oxidized in a non-oxidizing atmosphere or reducing atmosphere generally used and concentrate on surfaces to form oxides on surfaces of the steel sheet. The oxides reduce the wettability between the steel sheet surfaces and molten zinc to cause bare spots. The increase in concentration of each oxidizable element in steel sharply reduces the wettability to cause many bare spots. Even in the case where no bare spots are caused, the oxides are present between the steel sheet and a coating and therefore the adhesion of the coating is deteriorated. In particular, the addition of even a small amount of Si significantly reduces the wettability with molten zinc. Therefore, in galvanized steel sheets, Mn, which has a small influence on wettability, is often added. However, Mn oxides also reduce the wettability with molten zinc. Therefore, in the case of the addition of a large amount of Mn, a problem with the above bare spots is significant.
In order to cope with the problem, Patent Literature 1 proposes a method for improving the wettability of a surface of a steel sheet with molten zinc in such a manner that the steel sheet is heated in an oxidizing atmosphere in advance, the oxidation of an added element on the steel sheet surface by rapidly forming an Fe oxide film on the surface at a predetermined oxidation rate or more, and the Fe oxide film is then reductively annealed. However, when the oxidation of the steel sheet is significant, there is a problem in that iron oxide adheres to a roll in a furnace to cause scratches on the steel sheet. In addition, Mn forms a solid solution in the Fe oxide film and therefore is likely to form Mn oxides on the steel sheet surface during reductive annealing; hence, the effect of oxidation treatment is small.
Patent Literature 2 proposes a method in which a steel sheet is pickled after annealing, surface oxides are thereby removed, and the steel sheet is annealed again and is then galvanized. However, when the amount of an added alloying element is large, surface oxides are formed again during re-annealing. Therefore, even in the case where no bare spots are caused, there is a problem in that the adhesion of a coating is deteriorated.